1. Field of the Invention
The present invention relates to an OLED panel in which an organic light emitting device (OLED) formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to an OLED module in which an IC is mounted on the OLED panel. Note that, in this specification, the OLED panel and the OLED module are generically called light emitting devices. The present invention further relates to an electronic device using the light emitting device.
2. Description of the Related Art
An OLED emits light by itself, and thus, has high visibility. The OLED does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the OLED has no limitation on a viewing angle. Therefore, the light emitting device using the OLED has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. The OLED basically has a structure in which an anode/a light emitting layer/a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode/a hole injecting layer/a light emitting layer/a cathode are laminated in order or a structure in which an anode/a hole injecting layer/a light emitting layer/an electron transporting layer/a cathode are laminated in order.
In putting a light emitting device to practical use, a serious problem at present is a reduction in the luminance of the OLED, which is accompanied with deterioration of the organic light emitting material contained in the organic light emitting layer.
The organic light emitting material in the organic light emitting layer is easily affected by moisture, oxygen, light and heat, and the deterioration of the organic light emitting material is promoted by these substances. Specifically, speed of the deterioration of the organic light emitting layer is influenced by a structure of a device for driving the light emitting device, a characteristic of the organic light emitting material constituting the organic light emitting layer, a material for an electrode, conditions in a manufacturing process, a method of driving the light emitting device, and the like.
Even when a constant voltage is applied to the organic light emitting layer from a pair of electrodes, the luminance of the OLED is lowered due to the deterioration of the organic light emitting layer. Then, if the luminance of the OLED is lowered, an image displayed on the light emitting device becomes unclear. Note that, in this specification, a voltage applied to the organic light emitting layer from one pair of electrodes is defined as an OLED driving voltage (Vel).
Further, in a color display mode in which three kinds of OLEDs corresponding to R (red), G (green) and B (blue) are used, the organic light emitting material constituting the organic light emitting layer differs depending on the corresponding color of the OLED. If the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors, the luminance of the OLED differs depending on the color with the lapse of time. Thus, an image having a desired color can not be displayed on the light emitting device.
Furthermore, the luminance of the OLED has large temperature depending property, and thus, there has been a problem in that luminance of a display and a tone vary in accordance with the temperature in constant voltage drive.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a light emitting device in which a change of luminance of an OLED is suppressed and a desired color display can be stably performed even when an organic light emitting layer is somewhat deteriorated or when an environmental temperature is varied.
Between a light emission with a constant OLED driving voltage and a light emission with a constant current flowing through the OLED, the present inventor directs an attention to the fact that a reduction of the luminance of the OLED due to deterioration is smaller in the latter. Note that the current flowing through the OLED is called an OLED driving current (Iel) in this specification.
FIG. 2 shows a change of the luminance of the OLED between a case where the OLED driving voltage is constant and a case where the OLED driving current is constant. As shown in FIG. 2, the change of the luminance due to deterioration is smaller in the OLED with the constant OLED driving current. This is because not only an inclination of a straight line L-I becomes small but also a curve I-V itself moves to the lower side when the OLED is deteriorated (see FIGS. 15A and 15B).
Thus, the present inventor devised a light emitting device with a simple structure in which an OLED driving voltage can be corrected such that an OLED driving current is always kept constant even if the OLED driving current is varied due to deterioration or the like.
Specifically, in the present invention, a pixel portion for measuring the OLED driving current is provided in the light emitting device besides a pixel portion for displaying an image. It is preferable that the monitor pixel portion can display some images in order to be effectively used as a display portion. However, it is not essential that the monitor pixel portion can perform an image display. Hereinafter, in order to clearly distinguish between the above-described two pixel portions, the pixel portion in which an image display is aimed is called the display pixel portion (first pixel portion) and the pixel portion in which the measurement of the OLED driving current is aimed is called the monitor pixel portion (second pixel portion) through this specification.
The display pixel portion and the monitor pixel portion have the same structures of their respective pixels, and can be described with the same circuit diagrams. With regard to OLEDs of a pixel of the display pixel portion (hereinafter referred to as display pixel or first pixel) and a pixel of the monitor pixel portion (hereinafter referred to as monitor pixel or second pixel), the OLED driving voltages at the time when the luminance is maximum are controlled by a variable power supply, and both the voltages are preferably kept to have equivalent values.
Note that the variable power supply indicates a power supply in which a voltage supplied to a circuit or an element is not constant but variable in this specification.
Further, the light emitting device of the present invention includes a first means for measuring the OLED driving current of the OLED of the monitor pixel portion (hereinafter referred to as monitor OLED or second OLED), a second means for calculating a voltage applied to the OLED based on the measured value, and a third means for actually controlling the voltage value.
Note that the second means may be a means for comparing the current measured value and a reference value, and the third means may be a means for controlling the variable power supply to shorten a difference between the measured value and the reference value and correcting the OLED driving voltages of the OLED of the display pixel (hereinafter referred to as display OLED or first OLED) and the monitor OLED in the case where the difference exists.
The monitor pixel portion is input with a video signal of a different system from that of a video signal to be input to the display pixel portion. However, both the video signals are the same in the point that the signals each include gradation information, and only the system of an image to be displayed differs between the signals. Hereinafter, the video signal to be input to the display pixel portion is referred to as the display video signal and the video signal to be input to the monitor pixel portion is referred to as the monitor video signal.
When the OLED driving current of the monitor OLED is measured, an image for monitor (hereinafter referred to as monitor image) is displayed on the monitor pixel portion in accordance with the monitor video signal. The monitor image may be either a static image or a dynamic image. Further, the same gradation may be displayed on all the pixels. Moreover, it is preferable that the monitor image in which an average value in time is substantially the same between the OLED drive currents of the display OLED and the monitor OLED is displayed such that the degree of deterioration becomes the same between the display OLED and the monitor OLED.
Note that the reference value of the current does not need to be fixed at the same value at all times. A plurality of monitor images with different reference current values are prepared, and the monitor image may be selected every monitor. Of course, several kinds of monitor images with the same reference current value may be prepared.
With the above-described structure, in the light emitting device of the present invention, the reduction of the luminance of the OLED can be suppressed even with the deterioration of the organic light emitting layer. As a result, a clear image can be displayed.
Further, in the color display mode in which three kinds of OLEDs corresponding to R (red), G (green) and B (blue) are used, monitor pixel portions corresponding to the respective colors may be provided, and the OLED driving current may be measured for every OLED of each color to thereby correct the OLED driving voltage. With this structure, the balance of luminance among the respective colors is prevented from being lost, and a desired color can be displayed even when the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors.
Further, a temperature of the organic light emitting layer is influenced by an outer temperature, heat generated by the OLED panel itself, or the like. Generally, when the OLED is driven at a constant voltage, the value of the flowing current changes in accordance with the temperature. FIG. 3 shows a change of a voltage-current characteristic of the OLED when the temperature of the organic light emitting layer is changed. When the voltage is constant, if the temperature of the organic light emitting layer becomes higher, the OLED driving current becomes larger. Since the relationship between the OLED driving current and the luminance of the OLED is substantially proportional, the luminance of the OLED becomes higher as the OLED driving current becomes larger. In FIG. 2, the constant voltage luminance shows a vertical period for about 24 hours. This is because a temperature difference between day and night is reflected. However, in the light emitting device of the present invention, the OLED driving current can always be kept constant by the correction of the OLED driving voltage even if the temperature of the organic light emitting layer is changed. Therefore, a constant luminance can be obtained without being influenced by the temperature change, and also, the increase in power consumption with the temperature rise can be prevented.
Moreover, a degree of the change of the OLED driving current in the temperature change generally differs depending on the kind of the organic light emitting material. Thus, in the color display, the luminances of the OLEDs of the respective colors may be separately changed in accordance with the temperature. However, in the light emitting device of the present invention, the constant luminance can be obtained without being influenced by the temperature change. Thus, the balance of luminance among the respective colors is prevented from being lost, and a desired color can be displayed.
Incidentally, the present invention is particularly effective for an active matrix light emitting device of digital time gradation drive, and is also effective for an active matrix light emitting device of analogue gradation drive. Further, the present invention can be applied to a passive light emitting device.
Furthermore, the monitor pixel portion can be effectively used in a display of icons, logos, patterns, indicators and the like, and this can eliminate waste. In addition, the monitor takes the same type as the pixel, whereby the deterioration of the pixel OLED can be caught with higher definition. Thus, the luminance correction can be performed with ease and with accuracy.